Rails 5.2 HTTP2 Early Hint nopush support

[Ho Kuen Leung]

HTTP2 Early Hint support is added and will be released in 5.2

https://github.com/rails/rails/pull/30744

However there is no way to specify `nopush` in early hints when using javascript_include_tag / stylesheet_link_tag

Preload spec:

https://w3c.github.io/preload/?lipi=urn%3Ali%3Apage%3Ad_flagship3_pulse_read%3BwzEEHZvqToaWD2HAfSuSAw%3D%3D#server-push-http-2

This is necessary since there are servers with HTTP2 push support for early hints like Nginx

http://nginx.org/en/docs/http/ngx_http_v2_module.html#http2_push_preload

[Ho Kuen Leung]

Also for fonts / JS, the current helpers does not support adding `crossorigin` attribute

5.2.x helper source:

https://github.com/rails/rails/blob/5-2-stable/actionview/lib/action_view/helpers/asset_tag_helper.rb